1. Field of the Invention
The present invention is directed to an antenna array for magnetic resonance examinations of the type having array elements that are decoupled from each other and are independent, and which are arranged in two adjacent rows.
2. Description of the Prior Art
An antenna array of this type is described in an article by R. Jones with the title "Twelve Antenna Element Lower Extremity/Pelvic Array for MRI (A)", published in the conference volume for the ISMRM 1998 on page 440, "Proceedings of the International Society for Magnetic Resonance in Medicine, 6.sup.th Scientific Meeting and Exhibition, May 1998, Sidney, Australia." The antenna array is fashioned as a bilateral extremity array for the examination of the entire vessel system below the aorta bifurcation. In two rows, four saddle coil pairs are adjacently arranged with one coil per pair per row, which cover the normal length of the lower extremities of an adult. For decoupling, apart from a strictly symmetrical construction of the two rows, the saddle coil pairs of one row are arranged so as to be rotated 90.degree. relative to the other row.
The article by K. Y. Koyima et al.: "Lower Extremities: MR Angiography with a Unilateral Telescopic Phased-Array Coil" published in Radiology, vol. 196, No. 3, page 871 through 875, 1995 describes a unilateral telescopic phased-array-antenna with six saddle coils, which are sensitive to a linearly polarized field vector. The upper saddle coils are fashioned larger than the lower ones in order to achieve a good filling factor.
Not only the bilateral antenna array but also the unilateral antenna array has array elements with a linear antenna characteristic; this means that the primary sensitivity direction of the array elements extends along one single axis.
U.S. Pat. No. 5,430,378 discloses an antenna array with circularly polarizing array elements. Each array element is formed by two ring coils that are oriented perpendicularly to one another, whereby one ring coil is mounted in a base plate and the other ring coil is mounted on a carrier that is perpendicularly and symmetrically arranged relative to the base plate. The antenna array is fashioned for the examination of the lower extremities, the sensitivity area of the two ring coils of the individual antenna elements covering both extremities. Coupling ensues by a strictly symmetrical arrangement of the ring coils and by a partial overlap of neighboring ring coils.
The antenna array described in U.S. Pat. No. 5,548,218 also has circularly polarizing array elements that are fashioned for MR examinations of the lower extremities. Each array elements is formed by a ring coil and a butterfly coil. The middle area of the array elements is installed in a patient bed, whereas the outer areas are flexibly fashioned. This allows the outer elements to be "unfolded" away in order to position the patient. After the positioning, the outer areas are put on the patient, so that the entire lower extremities can be imagined with a high filling factor of the antenna. Here, the sensitivity area of the circularly polarizing array elements also encompassed both extremities.
The antenna array described in the U.S. Pat. No. 5,594,337 also has array elements whose sensitivity area encompasses both extremities.
Two decoupling modes are described in PCT Application WO 89/05115. The first mode is to arrange neighboring antenna elements in a partially overlapping manner. The second one transforms the input impedance of a connected pre-amplifier in a high-resistance manner.
U.S. Pat. No. 5,708,361 describes a way to decouple neighboring antenna elements, which does not require a specific geometrical arrangement of the array elements to be decoupled. The conductors of the array elements to be decoupled have an interruption (gap), and are connected in parallel to one another. The interruptions are short-circuited with at least one capacitor, with the decoupling being effected by a selection of the capacitance.
The antenna arrays described above enable a continuous display of the blood vessels in the lower extremities without repositioning the patient relative to the array. In comparison to the imaging of the blood vessels with a whole body resonator, a higher signal quality is therewith achieved. It is still desirable, however, to further increase the signal quality and therewith the signal-to-noise ratio in order to be able to also image finer blood vessels.